This invention relates to an injection mold, and more particularly to an injection mold for molding synthetic resin by injection molding.
In general, an injection mold used for an injection molding machine includes a mold body constituted by a first template and a second template joined to each other in a manner to be separable from each other. The mold body is formed therein with a cavity (a space in which molten resin is to be filled) of a configuration corresponding to that of an article to be molded, a resin flow passage through which molten resin is flowed therethrough toward the cavity, and a gate through which the molten resin flowed through the resin flow passage is injected into the cavity. At least one of the first and second templates is supported in a manner to be displaceable in a direction perpendicular to a parting face or line therebetween, so that relative displacement between the first template and the second template permits the mold body to be openably operated.
When molding is to be carried out, the first and second templates are abutted on an end surface thereof defined along the parting line against each other and then clamped together using any suitable clamping mechanism to close the mold body. Then, molten resin is injected through the gate into the cavity, resulting in being molded into an molded article. After the molding, the first and second templates are displaced in a direction in which they are separated from each other, so that the molded article may be removed from the mold body.
Now, the mold body of the conventional injection mold will be more detailedly described with reference to FIG. 11, which shows an essential part thereof. A mold body which is generally designated at reference numeral 1 in FIG. 11 is constructed so as to be slit or separated into first and second templates 2 and 3 along a parting line PL. The first and second templates 2 and 3 have end surfaces 2a and 3a defined along the parting line PL, respectively. When molding is to be carried out, the first and second templates 2 and 3 are joined to each other while keeping the end surfaces 2a and 3a abutted against each other. In the mold body 1 shown in FIG. 11, the first template 2 is formed therein with a cavity formation section 4a constituting a part of a cavity 4 and a sprue 6 for receiving therein molten resin 5 injected from a nozzle of an injection molding machine. The second template 3 is formed therein with a cavity formation section 4b constituting the remaining part of the cavity 4, a runner 7 through which molten resin introduced into the sprue 6 is guided along the parting line PL toward the cavity 4, and a gate 8 through which molten resin fed through the runner 7 is injected through a side surface of the cavity 4 into the cavity 4. In the mold body 1 shown in FIG. 11, the sprue 6 and runner 7 cooperate together to provide a resin flow passage through which molten resin is flowed toward the cavity 4.
The gate 8 arranged so as to inject resin from the side surface of cavity 4 along the parting line PL into the cavity 4 as described above is generally called a submarine gate or a tunnel gate.
The mold body shown in FIG. 11 is provided therein with an additional cavity on a right-hand side of the sprue 5 in order to concurrently produce a plurality of molded articles or moldings. For this purpose, an additional runner 7 for feeding resin to the additional cavity is connected to the sprue 6.
In the mold body thus constructed, the second template 3 is actuated by a pneumatic cylinder or the like, to thereby be displaced between a closed position at which the second template 3 is kept abutted or matched against the first template 2 as shown in FIG. 11 and an open position at which the former is kept separated from the latter. When molding takes place, the second template 3 is positioned at the closed position and tightly fastened to the first temperate 2 to a degree sufficient to withstand an injection pressure of molten resin by means of a clamping structure.
Injection molding using the mold body shown in FIG. 11 is carried out in a manner to inject molten resin 5 into the sprue 6 while clamping the first and second templates 2 and 3 together by means of the clamping mechanism to introduce the molten resin through the runner 7 and gate 8 into the cavity 4, resulting in a molded resin product 9 being obtained. The molded resin product 9 thus obtained includes an intended molded article 9a formed due to curing of molten resin filled in the cavity 4, a molded resin portion formed due to curing of molten resin remaining in the gate 8 (hereinafter referred to as "in-the-gate molded resin portion") which is designated at reference numeral 9b, a molded resin portion formed due to curing of resin remaining in the runner 7 (hereinafter referred to as "in-the-runner molded resin portion) which is indicated at 9c, and a molded resin portion formed due to curing of resin remaining in the sprue 6 (hereinafter referred to as "in-the-sprue molded resin portion") which is designated at 9d, which are connected to each other in order.
After injection of the molten resin into the mold body, the second template 3 is displaced to the open position to remove the in-the-sprue molded resin portion 9d from the sprue 6 and release the molded article 9a in the cavity from the cavity formation section 4a of the first template 2. Also, the molded article 9a in the cavity 4 is released from the cavity formation section 4b of the second template 3 by means of an ejector pin (not shown) and concurrently the molded article 9a is separated from the in-the-gate molded resin portion 9b.
Referring now to FIG. 12, an essential part of a mold body of another conventional injection mold is illustrated. The mold body generally designated by reference numeral 1 is so constructed that a first template 2 is provided therein with a runner 7 and a gate 8, which are arranged in a direction perpendicular to a parting line PL. The gate 8 is arranged so as to be orientated to a central position of a portion of a cavity 4 to which the gate 8 is connected, so that molten resin may be injected into the cavity 4 from one of both ends of the cavity opposite to each other in a direction perpendicular to the parting line PL. Such a gate arranged so as to permit resin to be injected into the cavity in the direction perpendicular to the parting line PL is called a pinpoint gate.
Molding using the mold shown in FIG. 12 is carried out in a manner to inject molten resin into the mold body 1 and then concurrently displace the first and second templates 2 and 3 in a downward direction in FIG. 12 while keeping an upper end (not shown) of an in-the-runner molded resin restrained, to thereby apply tensile force between a molded article 9a and an in-the-gate molded resin portion 9b, resulting in the in-the-gate molded resin portion 9b being separated from the molded article 9a and the in-the-runner molded resin portion 9c being removed from the runner 7 of the first template 2. Then, the first template 2 is stopped, so that only the second template 3 is moved in a direction indicated at an arrow in FIG. 12. This results in the molded article 9a being released from a cavity formation section 4a of the first template 2 and removed from a cavity formation section 4b of the second template 3 by means of an ejector pin (not shown).
Injection of resin into the cavity 4 by means of the submarine gate as shown in FIG. 11 permits the molded article 9a and in-the-gate molded resin portion 9b to be readily separated from each other at a boundary therebetween by shearing during release of the molded article 9a from the cavity 4, to thereby permit automatic operation of the injection molding machine to be readily carried out over all steps from injection of resin to release of the molded article.
However, the mold body provided with the submarine gate causes resin fragments to remain near an opening of the gate 8 on a side of the cavity 4, so that clogging of the gate with the resin fragments often occurs when the molded article 9a is separated from the in-the-gate molded resin portion 9b. Such clogging of the gate 8 fails to ensure satisfactory run of molten resin in the next molding operation, leading to a failure in molding such as surface sink or the like. Thus, the mold body provided with the submarine gate causes production of a certain number of defective molded articles, leading to a deterioration in yield of the products.
On the contrary, the mold body provided with the pinpoint gate as shown in FIG. 12 permits separation between the molded article and the in-the-gate molded resin portion during release of the molded article from the cavity to be accomplished by tensile force applied in an axial direction of the gate between the molded article and the in-the-gate molded resin portion rather than by shearing, to thereby effectively prevent clogging of the gate, resulting in a failure in molding such as surface sink or the like due to misrun of molten resin from being substantially prevented.
Unfortunately, the mold body provided with the pinpoint gate causes the in-the-gate molded resin portion 9b to be separated from the molded article 9a in a manner to be torn off from the molded article 9a; so that separation of the in-the-gate molded resin portion 9b from the molded article 9a while keeping resin at a central portion of the gate 8 insufficiently cured causes the uncured resin at the central portion of the gate to remain on a side of the molded article 9a in a threading or roping manner, resulting in the residual resin forming a large protrusion 9a1 on an outer surface of the molded article 9a.
In order to eliminate the disadvantage, the mold body provided with the pinpoint gate employs an approach wherein a cycle of operation of an injection molding machine is set so as to permit the in-the-gate molded resin portion 9b to be separated from the molded article 9a after a period of time for cooling required for sufficiently curing resin at the central portion of the gate elapses, to thereby prevent the protrusion 9a1 increased in size from being formed on the molded article 9a.
However, such setting of operation cycle of the molding machine still causes resin at the central portion of the gate to be insufficiently cured irrespective of a lapse of time, resulting in the protrusion 9a1 increased in size being formed on the molded article 9a during separation of the in-the-gate molded resin portion 9b from the molded article 9a; because pollution of an inner surface of the gate due to repeating of the molding operation deteriorates heat dissipation through the inner surface of the gate, to thereby increase a period of time required for curing resin at the central portion of the gate.
Thus, the conventional mold body provided with the pinpoint gate must be cleaned frequently or at reduced intervals, to thereby cause maintenance of the mold to be required at reduced or frequent intervals, leading to a deterioration in productivity thereof.